Apparatus utilizing various types of media such as optical disks, magneto-optical disks, and flexible magnetic disks are known as disk drive apparatus. Among such apparatuses, hard disk drives (HDDs) have widely spread as storage devices for computers and have become one of indispensable storage devices in current computer systems. Further, thanks to their advantageous characteristics, the use of HDDs is increasing in various fields such as movie image recording/reproducing apparatus, car navigation systems, and removable memories used with digital cameras, and computers.
An HDD for recording and reproducing data with a head element unit exercises positioning control over the head element unit based on servo data formed on a magnetic disk. Each of the tracks concentrically formed on a magnetic disk, has a plurality of servo sectors, and each of the servo sectors is constituted by servo data and user data.
A goal in developing HDDs is to provide an increased capacity for data storage capacity. The perpendicular magnetic recording method has been proposed as a technique for improving the recording density of a magnetic disk. According to the perpendicular magnetic recording method, recording is performed with recording magnetization oriented in a direction perpendicular to a recording surface. perpendicular magnetic recording allows high stability of recording to be achieved at a high recording density because magnetic fields are subjected to a low degree of demagnetization by adjacent magnetic domains as the recording density increases unlike that in in-plane magnetic recording in which the direction of recording magnetization is parallel to a recording surface.
In the case of perpendicular magnetic recording, however, it is known that a DC erase area (DC magnetized area) on a recording surface adversely affects data reading performed by a read element. A magnetic field in the DC erase area constitutes a disturbance to the read element, and the SER of a read signal is significantly decreased. Under the circumstance, it is proposed in Japanese Laid-Open Patent No. 2002-230734 (“Patent Document 1”) to perform AC erase on a user area as whole including areas between data tracks.
When a DC erase area exists between data tracks, a magnetic field from that area constitutes a disturbance to the read element, which results in a significant reduction in SER (Soft Error Ratio). Therefore, it is specifically proposed to perform AC erase (AC magnetization) on such an area using a servo writer before servo data is written or at a process of writing servo data on a recording surface.
However, when a servo write process employs self servo write, AC erase may be disabled before the servo write or during the servo write process. Self servo write is to write a pattern using a head element unit of an HDD and to write a new pattern by positioning and timing the head element unit while reading the pattern with the head element unit.
A timing pulse to serve as a reference for the write timing measurement and a radial pattern for positioning the head element unit are written on the recording surface by the self servo write, but accurate detection of those patterns is sometimes disabled by magnetization of their surroundings on which AC erase has been performed. It is required to perform AC erase in a desired area of a recording surface not only when a servo write technique is used but also when an AC erase process is performed on the recording surface after writing servo data on the recording surface.